The present invention relates to electrical discharge machining and more particularly to an electrode tool in the form of a continuous metallic filament such as a wire or tape provided with an electroplated surface of particular composition.
The machining efficiency of an electrode tool used for machining a metallic workpiece by electrical discharges is closely related to an appropriate choice of diverse machining parameters, such as for example the machining gap, the machining fluid flow rate, the machining current intensity, the wave form and frequency of the electrical discharges. Although the machining efficiency can be greatly improved by an adaptive adjustment of those machining parameters, it nevertheless reaches a limit which depends upon the galvanic pairs formed by the materials of the electrode tool and of the electrode workpiece.
In addition, the machining efficiency of a metallic filament such as a wire or tape used as an electrode tool for cutting a workpiece by electrical discharges is dependent to a great degree on the current density which the wire or tape is capable of accepting. The machining accuracy depends upon the accuracy of alignment of the wire electrode between its guiding and support members while being translated through the machining zone. The alignment of the wire electrode tool is influenced by the traction or pull exerted on the wire, with the result that the wire must be capable of being subjected to a high current density as well as being endowed with a high mechanical strength under traction. Such requirements are uncompatible when they are applied to conventional metallic wires because materials which are good conductors of electrical current have a very poor mechanical strength
The present invention permits a filament electrode such as a wire electrode to be obtained for EDM which satisfies both conditions of high electrical efficiency and high mechanical strength. In addition, the invention has for an object to provide an electrode which greatly facilitates the triggering of the electrical discharges and which decreases any tendency to cause short circuits, with the result that the wire electrode of the invention enables machining at higher speeds than is the case with conventional wire electrodes.
A filament electrode such as a wire electrode according to the present invention is characterized by being provided with an active surface comprising at least 50% by weight of one or more of the following metals: zinc, cadmium, tin, lead, antimony and bismuth.
The active surface of the wire electrode consists of a coating protecting the core of the wire against thermal shocks caused by the electrical discharges resulting, from the low temperature of vaporization of the above-mentioned metals. For all practical purposes, the active surface coating provides a thermal shield for the wire which in turn permits an increase in the power of the electrical discharges or their frequency without risking rupturing the wire electrode in the machining zone.
The surface coating may have a thickness in the range of 1 to 25 microns according to the intensity and direction of the electrical discharges. The coating may further consist, for example, of at least 80% cadmium, which presents the further advantage of considerably reducing the coefficient of friction relative to the wire guiding surfaces, and of greatly facilitating cold sizing of the wire.
The mechanical strength of the wire and the intensity of the current flow through the wire can further be greatly increased by providing a wire having a steel core surrounded by a layer of copper or silver provided in turn by the protective thermal coating consisting of, for example, zinc, cadmium, tin, lead, antimony or bismuth, or alloys thereof.
A further advantage derived from providing a wire electrode with a surface coating according to the invention is to be able to convert into machining discharges the discharges occurring during a short circuit, because the metallic bridges formed in the machining gap between the wire electrode and the workpiece are very easily melted by the heat generated by the high intensity current of the short circuit. Metals having low inherent vaporization heat characteristics are therefore more particularly advantageous for such an application.
The present invention is addressed to a method and to an apparatus for manufacturing the electrode of the invention.